1. Field of the Invention
This invention relates to a process and catalyst for the conversion of hydrocarbons, and more specifically for the aromatization of gasoline-range hydrocarbons.
2. General Background
The catalytic reforming of hydrocarbon feedstocks in the gasoline range is an important commercial process, practiced in nearly every significant petroleum refinery in the world to produce aromatic intermediates for the petrochemical industry or gasoline components with high resistance to engine knock. Demand for aromatics is growing more rapidly than the supply of feedstocks for aromatics production. Many large aromatics complexes have been completed recently or are under construction or planned which emphasize high yields of paraxylene as feedstock for polyester production to serve this burgeoning market. The catalytic reforming unit therefore must operate more efficiently at higher severity in order to meet these increasing needs for chemical aromatics while conserving feedstocks. This trend creates a need for more effective aromatization processes and catalysts.
Catalytic reforming generally is applied to a feedstock rich in paraffins and naphthenic hydrocarbons and is effected through diverse reactions: dehydrogenation of naphthenes to aromatics, dehydrocyclization of paraffins, isomerization of paraffins and naphthenes, dealkylation of alkylaromatics, hydrocracking of paraffins to light hydrocarbons, and formation of coke which is deposited on the catalyst. Increased aromatics and gasoline-octane needs have turned attention to the paraffin-dehydrocyclization reaction, which is less favored thermodynamically and kinetically in bifunctional reforming than other aromatization reactions. Considerable leverage exists for increasing desired product yields from aromatization by promoting the dehydrocyclization reaction over the competing hydrocracking reaction while minimizing the formation of coke.
The effectiveness of aromatization catalysts comprising a non-acidic L-zeolite and a platinum-group metal for dehydrocyclization of paraffins is well known in the art. The use of these aromatization catalysts to produce aromatics from paraffinic raffinates as well as naphthas has been disclosed. Commercialization has been slow and is limited in scope in light of special pretreating required to obtain the relatively high selectivity to aromatics that this technology features. Further, such catalysts yield a high proportion of benzene and toluene compared to the more desired xylenes. Thus, there is a particular need for further improvements in selectivity as well as activity and stability of such dehydrocyclization catalysts.
The art discloses reforming with a broad range of catalysts containing large-pore zeolites and Group VIII metals. U.S. Pat. No. 4,104,320 (Bernard et al.) discloses dehydrocyclization with potassium-form L-zeolite charged with one or more dehydrogenating metals of Group VIII and another metal such as rhenium, iridium, tin or germanium. Bernard et al. teach that the other metal preferably is introduced at the same time as platinum or palladium and does not suggest such metals in the framework of the L-zeolite.
U.S. Pat. No. 4,990,710 (Dessau et al.) teaches dehydrogenation to yield aromatics using a tin-modified microporous crystalline silicate. U.S. Pat. No. 5,518,708 (Skeels et al.) teaches a molecular sieve having tin or chromium as framework tetrahedral oxide units, with zeolites Y and L being among the disclosed sieves. The use of such sieves is disclosed generally for a plethora of hydrocarbon-conversion, separation and oxidative combustion processes.
None of the above references discloses aromatization with a catalyst containing a bound nonacidic large-pore molecular sieve containing framework tin and a platinum-group metal component.